Method of manufacturing electric heating plates



P 1941- K. FISCHER 2,255,209

METHOD OF MANUFACTURING ELECTRIC HEATING PLATES Filed June 10, 1939 INVENTOR KARI. FISCHER BY m ATTORNEYS Patented Sept. 9, 1941 METHOD OF MANUFACTURING ELECTRIC HEATING PLATES Karl Fischer, Oberderdingen, Wnrttemberg, Germany Application Jul. 10, 1939, Serial No. 278,447

2 Claims.

. The present invention relates to a process for embedding resistance heating wires of coiled form in the grooves provided in the undersldes of electric heating plates. There are many known processes for effecting this operation. Hitherto, however, none of the heating plates produced according to these processes satisfied requirements as regards quality.

The problem which must be solved in the production of grooved heating plates consists in that the Joules heat produced in the heating wires has to be transferred as quickly as possible and with the minimum amount of loss to the grooved heating plates through the insulating material, which must necessarily be arranged between the heating wires andzthe heating plate. In order to obtain the best possible transfer of heat the fundamental-object is to use an insulating material with the highest possible coefiicient of heat conductivity. This provision is, however, limited by the property of the material in as much as generally, materials with a bad electrical conductivity also have a bad heat conductivity. Obviously the thickness of the insulating layer between the heating wire and the heating plate must be chosen as thin as possible, but in this connection there are considerable technical difllculties to be overcome, which has not yet proved possible without incurring other disadvantages (insufiicient transfer of heat by contact at the touching faces). It is necessary to provide between the heating wire and the insulating material on the one hand as well as between the insulating material and the heating plate on the other hand the most intimate possible mechanical contact so that the transfer of heat at these faces is as great as possible.

The solution of the above mentioned problem consists in a compromise to be chosen according to the points of view of practise between the factors electrical permanent insulating capacity and heat conductivity ofthe insulating mass, as well as the thickness of the insulating layer between heating wire and the heating plate on the one hand and the loss due to current leakage on the other hand whilst the production of a perfect mechanical contact in the final state between heating wire, insulating material and the heating plate is primarily dependent upon the embedding process and/or the physical properties of the insulating material.

The embedding process according to the inven-.

tion exhibits a considerable advantage over the 7 present state of the art in that the heating wires Germany March 3, 1937 terial in the heating grooves, which is still loose and powdery, without the use of supporting apparatus or similar devices whilst after providing a further loose powdery insulating layer the whole is pressed, so that the insulating material, which consequently is compressed and has its volume substantially reduced, is brought into perfect mechanical contact in a single pressing process with the heating groove walls and all parts of the heating windings, and is not destroyed by subsequent drying or burning. This embedding material, which is homogeneous throughout, uniformly packed and contacts with the heating windings as well as the walls and base of the grooves cannot be produced according to any of the known processes although even with these one or more uses is made of pressure.

For example, a process is known in which the insulating material mixed as a paste is pressed in at high pressure by a stamp corresponding exactly to the groove walls so that a uniformly thick wall and base coating is produced which is then dried by gradually increasing heating. The compression produced by the pressure of the insulating material mixed to a paste with water is limited by the incompressibility of the liquid mixing medium. After the coating of the heating grooves (which is as uniformly thick as possible) produced in this way is dried and hardened, the coiled heating element is inserted in the grooves provided in this way with a hard coating. The grooves and wire therein are now covered with a correspondingly thick layer of the same embedding material, whereon the disc .is further treated with gradually increasing temperature and gradually increasing pressure until complete drying is eii'ected.

Apart from other faults with this process it is never possible to produce a uniform pressure around the heating windings and consequently a perfect mechanical contact cannot be obtained; in this process the wall coating, which is pressed Y and dried in the first operating process and is therefore hard, is subjected to aconsiderable pressure which causes cracking of this hardened ceramic groove inset. It is moistened again which is not favourable for hardened ceramic materials whilst no intimate homogeneous combination takes place between the embedding material hardened by drying and the freshly imposed embedding material. As the embedding material used at the beginning of the second working operation is placed on the upper side of the inserted heating spirals, on the underside of can be inserted by hand inv the insulating mathe heating spirals where the greatest transfer of touching face.

heat takes place and where there should therefore be the most intimate contact between the heating wire surface and the embedding material. no such combination exists since the still plastic embedding material can neither reach the inside of the heating spirals nor to the base of the channel of the hard insulating material, that is, between this and the under side of the heating spirals.

In another known process the heating grooves are likewise provided with a hard coating of insulating material effected by hydraulic pressure before the insertion of the heating winding so that there remains a narrow slot which serves to receive a vertical zig-zag heating winding. The heating winding is then inserted by hand and a covering layer is pressed on. With the double layer of insulating material provided in the heating grooves in two successive pressing operations in this form there would still remain an amount of inhomogeneity in the touching face which would be harmful to the transmission of "heat even if the insulating material was packed equally tightly on both sides of this This is, however, not the case since in this process also the side of the heating wire which should have an intimate contact with the insulating material in order to give a transfer of heat as free as possible from loss,

rests on the insulating layer hardened through the first pressing and an insulating mass forming a covering layer and giving a certain degree of good contact with the heating wire is only imposed subsequently from the side of the heating winding directed away from the direction of the desired heat current. n the side surfaces of the heating wire, however, and in particular on the part of the heating wire directed below the heating disc there is no intimate contact of the insulating mass with thewire.

The process according to the present invention consists essentially in filling the grooves with loose powdered insulating material, forming a furrow in the insulating material in each groove whilst consolidating the material to only geneous pressure, the last traces of air still remaining in the insulating material can escape through the recess of the grooved pressing stamp lying on the upper edges of the grooves.

It is preferable to give the grooves an approximately rectangular cross. section so as to provide the maximum of surface to which the insulating mass in the heating grooves can adhere and to provide'the maximum amount of insulating material for supporting the heating wires.

The process is illustrated by way of example in the form of construction shown in the accompanying drawing and in which:

Fig. l is a vertical section through the heating disc at the beginning of the process, showing the insulating material which has been shaken on.

Fig. 2 shows the same section together with the furrow forming stamp,

Fig. 3 is a. section through a heating groove after the forming of the furrow (on double the scale of Fig. 2),

Fig. 4 is a vertical section according to Fig. 2

after the introduction of the heating spirals and the imposition of further insulating material,

Fig. 5 is a vertical section according to Fig. 4 showing the plane packing stamp after the termination of the main pressing,

Fig. 6 is a vertical radial section according to Fig. 5 showinga channelled stamp after the terminationof the pressure.

The heating disc I carries on its under side ribs 2, which are preferably cast on, and between the sides of which and the bottom of the disc there are grooves 3 of approximately rectangular cross section serving to receive the resista slight extent adjacent the open end of the grooves, inserting the heating wires in the furrows, imposing a further quantity of loose pow-- dered insulating material and then compressing the whole insulating mass in one pressing operation.

By the present process the heating spirals penetrate by themselves into the non-compressed,

or only slightly compressed loose insulating material between the heating ribs and to such a depth that with the pressure following after providing the further layer of loose insulating material, the heating spirals, which still remain at the same distance from the walls of the heating grooves, together with the insulating material which is being compressed, are pressed downwardly whilst at the same time insulating material penetrates to the inside of the heating spirals fmm all sides and especially from the underside and is compressed in the same way as the other insulating material in the heating grooves.

ings are exposed to air and quickly oxidised. In

this last pressing, which is effected under homoance heating-wires of coiled form. To start the process the powdered insulating material 4, at the most only slightly moistened, is placed thereon by sifting or like means until the upper edges of the ribs 2 are covered with a layer of a certain depth. The heating disc is then shaken so that the insulating material 4 settles down as equally as possible and with a uniform density in the heating grooves 3. Any insulating material l projecting over the upper edges" of the ribs 2 is removed by means of a scraper or the like.

The upper layer of the insulating material 4 which is in the'heating grooves 3 and extends up to the edges of the'same is divided into two flanking masses (see Fig.- 3) by the application of a furrow-forming stamp 5 which has on its under side wedge shaped ridges l2 coinciding with the centre lines of the heating grooves and having concave faces. In particular the lower layers of the insulating material 4 in the grooves 3v maintain, with this furrow formation effected by the lateral movement of the upper layers, sub.

stantially the same density as was produced by the shakingprocesswhilsttheflanksofthefur- 5 rows I formed in the mass of insulating material by the furrow-forming stamp 5, while only sligh ly consolidated, at the same time form I ridges I2 01 the iurrow-iormingstamp I is such that the heating coils I after the termination oithe pressing operation sink to a well defined extent in the compressed insulating material, which is correspondingly decreased in volume, and iinally remain resting at a certain predetermined distance from the base oi the heating grooves. as shown him. 5. The entire embedding material is so compressed by the application of the pressing stamp I that it only overtops slightly the-upper borders of the ribs 2 and afterwards it is still further compressed by the application of the grooved stamp ll having recesses ll atthosepointswhichrestonthe upperbordersoitherihstsotbattheembedding material in each heating groove no longer remains connected with the embeddingmaterial in the adjacent grooves. a

ll areprovidedinthe groovedstamp ll whereby any airstillremainingintheembedding material can iinallyescape. which is not possible, or only incompletely so, in the plane preuing stamp I. The amount or the height of thesecondlayeroiinsulatlngmaterlal (see! inl'igii) isoisuchdimensionsthatatterpress- 'ing with the-stamps I and ll the maximum de-' substantial percentage addition, preferably more than 0! its entire volume, of finely ground particles or a ram nature.

Having now described my invention, what 3. claim and desire to secureby Letters Patent is: 1. A method of manufacturing grooved electric heating plates, said method comprising pouring a comparatively loom comminuted insulating materiaLinto the grooves of a plate to iill said grooves with said material, forming a wedgeshaped iurrow with its base at the top and with upwardly convex flanks in the insulating material in each groove without compressing the material at the bottom and at th sides 0! the groove and-while consolidating to a slight extentv only that part or the material which is situated adjacent to the open end of the grooves, inserting a heating wire in the-furrow, covering the wire with an additional quantity of loose comminuted insulating material, and then compressing the entire insulating material in one pressing operation.

2. A method of manufacturing grooved electric heating plates, said method comprising pouring a comparatively loose comminuted insulating materlalintothcgroovesotaplatetoiillsaid grooves with said material, i'orming a wedgeshapedmrrowwithitshascatthetopandwith upwardly convex flanks in the insulating material ineachgrooveto divide theinsulating material along the center or each groove without compressing the material at the bottom and at the side-sot the grooveandwhile consolidatingtoa slight extent only that part at the material which issituatedadiacenttotheopenendotthe grooves,insertingaheatingwireinthe furrow. and then, after applying a further quantity oi loose powdered insulating material, compressing the entire insulating material in one pressing gperationuntilitiillssuhstantiallytheentire KARL 

